This invention relates to optical systems and more particularly to apparatus and methods for the conversion of acoustic or other mechanical wave energy into an optical pattern.
There has been developed an acoustical-to-optical conversion cell which is highly efficient and exhibits good response over a fairly broad frequency range. Representative cells of this type are disclosed, for example, in John F. Dreyer U.S. Pat. No. 3,597,043 granted Aug. 3, 1971, and in Pal Greguss U.S. Pat. No. 3,831,434 granted Aug. 27, 1974. Such cells customarily employ a thin film of a nematic liquid crystal compound which is supported between a pair of transparent or translucent glass plates. The cell is illuminated by polarized light and is viewed through a polarizing filter to provide a real-time visual image of an acoustic wave pattern incident thereon. As used herein, the terms "acoustic", "sonic", etc., are intended to include mechanical energy having a frequency both within and outside the audio frequency spectrum.
The acoustical-to-optical conversion cells employed heretofore have exhibited certain disadvantages. For example, the sensitivity of the cells often varied widely depending upon such factors as their thickness and on the frequency of the applied mechanical wave. In cases in which a resonance condition was created, extremely good sensitivity was observed, but for a cell of uniform response characteristics such resonance existed only over a fairly narrow frequency band.
To achieve optimum resonance the thickness of one of the glass plates plus that of the liquid crystal film should be equal to one-half wavelength (or a whole number multiple thereof) of the applied acoustic energy. In cases in which such frequency matching does not occur, it was often necessary heretofore to substitute a cell of different thickness or to vary the applied frequency in order to achieve a satisfactory image of the acoustic pattern. When attempts were made to tilt the cell relative to the applied ultrasonic beam in an effort to achieve such matching, the pattern was distorted and in many instances was difficult to properly interpret.